The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. Furthermore, all embodiments are not necessarily intended to solve all or even any of the problems brought forward in this section.
Radio Frequency (RF) systems generally comprise an antenna, connected to a RF receiver and to a RF transmitter via a Front-End Module (FEM). For example, the RF receiver may comprise a Low Noise Amplifier (LNA) and the RF transmitter may comprise a Power Amplifier (PA).
RF receivers have to maintain their performance even when receiving high-power blocking signals at the antenna. This is generally the case in Frequency Division Duplex (FDD) systems where the RF transmitter sends a high-level signal while the RF receiver receives a low-level signal. Usually the blocking signals are filtered prior to the Low Noise Amplifier (LNA) of the RF receiver, and sometimes after the LNA.
In some cases, an intermodulation product between an incoming blocking signal and the own transmission signal leakage (due to finite isolation between amplifiers PA and LNA) may fall into the frequency bandwidth of the low-level reception signal. In such cases, the RF receiver sensitivity is jeopardized and the performance is below requirements.
These scenarios may be very frequent depending on the transmission-reception frequency separation and on the blocking signal spectral position. The levels of the blocking signal are specified by the standardization organization of the targeted RF communication system.
Moreover, in most modern mobile communication devices, the RF system, i.e. the cellular Radio Frequency Integrated Circuit (RF-IC), is coexisting with other radio engines, e.g. Bluetooth and Wireless LAN. In this case, the blocking signal is coming from a transmitter exhibiting thereby very high power levels. Even with good antenna isolation between radio engines and good RF filtering performance between the cellular antenna and the input of the reception path, intermodulation products may completely desensitize the receiver.
To overcome this coexistence issue a highly linear RF front-end (LNA and mixer) must be designed.
At the present time, existing RF CMOS front-end solutions are using inductors for output load and for feedback loops to achieve a high linearity in LNA circuits. Such components require large areas to be integrated properly with good quality factors.
Other solutions use thick oxide MOS devices along with above nominal process supply voltage. Such devices require an additional process mask during fabrication which significantly increases the cost of the overall processing in mass production. Also they slightly increase the overall power consumption of the RF front-end.
Other solutions comprise passive transferred impedance filtering at the LNA output. Such technique cannot work efficiently with feedback LNA as input impedance at blocker frequency will increase and voltage swing at the amplifier input accordingly. Besides, such filter requires large capacitor area to work properly along with additional RF phase generation.
U.S. Pat. No. 6,288,609 describes a 2-stage LNA with an Automatic Gain Control (AGC) system that commands the gain of the 2nd stage (VGA) and the bias of the 1st stage (amplifier feedback stage) to achieve gain control and linearity enhancement respectively. The total LNA gain does not remain constant when linearity enhancement is activated, and, as a consequence, noise performance is decreased.
U.S. Pat. No. 7,746,169 describes a LNA with two possible modes of operation: a distortion cancellation mode (PDC) for linearity improvement and a high-gain mode for sensitivity case. To improve LNA linearity, additional active devices are used to cancel 3rd order intermodulation products generated by the main active device. Moreover, the gain is decreased and as a consequence the power consumption will increase. The PDC has a gate-to-source voltage dependency and is sensitive to device mismatch. Therefore, it requires a proper calibration method to achieve the linearity improvement.
U.S. Pat. No. 7,266,360 describes an RF amplifier that includes a parasitic capacitor cancellation system, an image frequency filtering section and a gain controllable section. As in the systems described above, the total LNA gain does not remain constant, and, as a consequence, noise performance is decreased.
There is a need for improved methods and devices for enhancing linearity of an amplifier while maintaining the overall gain and power consumption of the amplifier constant.